Xpath to XQuery

1. Xpath to XQuery February 23rd, 2004

2. Other Stuff • HW 3 is out. • Instructions for Phase 3 are out. • Today: finish Xpath, start and finish Xquery. • From Wednesday: dbms internals.

3. Xpath: Wildcard Result: <first-name> Rick </first-name> <last-name> Hull </last-name> * Matches any element //author/*

4. Xpath: Attribute Nodes /bib/book/@price Result: “55” @price means that price is an attribute

5. Xpath: Predicates /bib/book/author[firstname] Result: <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author>

6. Xpath: More Predicates Result: <lastname> … </lastname> <lastname> … </lastname> /bib/book/author[firstname][address[//zip][city]]/lastname

7. Xpath: More Predicates /bib/book[@price < “60”] /bib/book[author/@age < “25”] /bib/book[author/text()]

8. Xpath: Summary bib matches a bib element * matches any element / matches the root element /bib matches a bib element under root bib/paper matches a paper in bib bib//paper matches a paper in bib, at any depth //paper matches a paper at any depth paper|book matches a paper or a book @price matches a price attribute bib/book/@price matches price attribute in book, in bib bib/book/[@price<“55”]/author/lastname matches…

9. Comments on XPath? • What’s good about it? • What can’t it do that you want it to do? • How does it compare, say, to SQL?

10. XQuery • Based on Quilt, which is based on XML-QL • Uses XPath to express more complex queries

11. FLWR (“Flower”) Expressions FOR ... LET... WHERE... RETURN...

12. XQuery Find all book titles published after 1995: FOR$xINdocument("bib.xml")/bib/book WHERE$x/year > 1995 RETURN { $x/title } Result: <title> abc </title> <title> def </title> <title> ghi </title>

13. XQuery Find book titles by the coauthors of “Database Theory”: FOR$xINbib/book[title/text() = “Database Theory”]/author$yINbib/book[author/text() = $x/text()]/title RETURN <answer> { $y/text() } </answer> Result: <answer> abc </ answer > < answer > def </ answer > < answer > ghi </ answer > The answer willcontain duplicates !

14. XQuery Same as before, but eliminate duplicates: FOR$xINbib/book[title/text() = “Database Theory”]/author$yINdistinct(bib/book[author/text() = $x/text()]/title) RETURN <answer> { $y/text() } </answer> Result: <answer> abc </ answer > < answer > def </ answer > < answer > ghi </ answer > distinct = a function that eliminates duplicates

15. XQuery: Nesting For each author of a book by Morgan Kaufmann, list all books she published: FOR$aINdistinct(document("bib.xml")/bib/book[publisher=“Morgan Kaufmann”]/author) RETURN <result> { $a, FOR$tIN /bib/book[author=$a]/title RETURN$t} </result>

16. XQuery Result: <result> <author>Jones</author> <title> abc </title> <title> def </title> </result> <result> <author> Smith </author> <title> ghi </title> </result>

17. XQuery • FOR$x in expr -- binds $x to each value in the list expr • LET$x = expr -- binds $x to the entire list expr • Useful for common subexpressions and for aggregations

18. XQuery <big_publishers> FOR$pINdistinct(document("bib.xml")//publisher) LET$b := document("bib.xml")/book[publisher = $p] WHEREcount($b) > 100 RETURN { $p } </big_publishers> count = a (aggregate) function that returns the number of elms

19. XQuery Find books whose price is larger than average: LET$a=avg(document("bib.xml")/bib/book/price) FOR$b in document("bib.xml")/bib/book WHERE$b/price > $a RETURN { $b } Let’s try to write this in SQL…

20. XQuery Summary: • FOR-LET-WHERE-RETURN = FLWR FOR/LET Clauses List of tuples WHERE Clause List of tuples RETURN Clause Instance of Xquery data model

21. FOR v.s. LET FOR • Binds node variables iteration LET • Binds collection variables one value

22. FOR v.s. LET Returns: <result> <book>...</book></result> <result> <book>...</book></result> <result> <book>...</book></result> ... FOR$xINdocument("bib.xml")/bib/book RETURN <result> { $x } </result> LET$xINdocument("bib.xml")/bib/book RETURN <result> { $x } </result> Returns: <result> <book>...</book> <book>...</book> <book>...</book> ... </result>

23. Collections in XQuery • Ordered and unordered collections • /bib/book/author = an ordered collection • Distinct(/bib/book/author) = an unordered collection • LET$a = /bib/book $a is a collection • $b/author  a collection (several authors...) Returns: <result> <author>...</author> <author>...</author> <author>...</author> ... </result> RETURN <result> { $b/author } </result>

24. Collections in XQuery What about collections in expressions ? • $b/price list of n prices • $b/price * 0.7  list of n numbers • $b/price * $b/quantity  list of n x m numbers ?? • $b/price * ($b/quant1 + $b/quant2) $b/price * $b/quant1 + $b/price * $b/quant2 !!

25. Sorting in XQuery <publisher_list> FOR$pINdistinct(document("bib.xml")//publisher) RETURN <publisher> <name> { $p/text() } </name> , FOR$bIN document("bib.xml")//book[publisher = $p] RETURN <book> { $b/title , $b/price } </book> SORTBY(priceDESCENDING) </publisher> SORTBY(name) </publisher_list>

26. If-Then-Else FOR$h IN //holding RETURN <holding> { $h/title, IF$h/@type = "Journal" THEN$h/editor ELSE$h/author } </holding> SORTBY (title)

27. Existential Quantifiers FOR$b IN //book WHERESOME$p IN $b//paraSATISFIES contains($p, "sailing") AND contains($p, "windsurfing") RETURN { $b/title }

28. Universal Quantifiers FOR$b IN //book WHEREEVERY$p IN $b//paraSATISFIES contains($p, "sailing") RETURN { $b/title }

29. Other Stuff in XQuery • BEFORE and AFTER • for dealing with order in the input • FILTER • deletes some edges in the result tree • Recursive functions • Currently: arbitrary recursion • Perhaps more restrictions in the future ?